1. Field of the Invention
The present invention relates generally to a reproduction technique, and specifically to a method and apparatus for using spectral sensors for media identification and improved scanner correction.
2. Brief Description of Related Developments
Color correction for scanners is a problem in color management systems. Scanners sense color in a different way than the human visual system, hence technology is needed that relates scanner signals and colorimetric (i.e. visual) signals. Scanners are deviant from the human visual system in ways that differ depending on the spectral characteristics of the medium (i.e. substrate and inks, or a live scene) being scanned. To address this problem, different characterizations, or scanner profiles are built for different media. In conventional scanners this may be accomplished by scanning in a scanner target containing patches with known colorimetric values of those patches measured by a reference spectrophotometer, extracting the scanner red, green and blue (RGB) signals for each of the patches, and building a profile that maps scanner RGB to a colorimetric space. This process is repeated for each different medium. Two problems exist with such a solution.
First, the aforementioned method leaves the user with the burden of selecting the correct profile for processing a given scan. In a recent study, over half of users asked to identify the medium by examining prints from photographic, offset, and inkjet printers were not able to do so correctly. Considerable error arises from choosing the incorrect scanner profile.
Second, even when the correct profile is chosen, there can be sufficient variation in input materials within a given media class to give rise to objectionable errors in scanner color correction. An example of the latter, if the medium is known to be photographic, the scanner""s response can change considerably depending on the manufacturer of the medium, (e.g. Kodak vs. Fuji), and can vary from batch to batch within the same manufacturer.
U.S. Pat. No. 5,450,165 discloses one method to measure the optical density of toner on a photoreceptor in areas where the color separated image has constant pixel values at least as large as the aperture of the sensing system. This method correlates areas with pre-existing image data xe2x80x98incidental test patchesxe2x80x99 for print quality measurements. The incidental test patches are predetermined from the stored image file and printed by the system.
U.S. Pat. No. 5,481,380 discloses a method of scanner calibration where a test target containing patches of known colorimetric values is scanned, the resulting scanner RGB values are transformed into estimates of colorimetric values using a simple scanner model, and the estimates are then refined with the known measurements. Since this approach only derives the correction based on a factory installed test target, and does not adjust the correction for each input scan, the technique does not address either of the aforementioned problems.
Using the approach in U.S. Pat. No. 5,481,380, the user can read different test targets for different print samples (i.e. test target #1 for lithographic print, test target #2 for xerographic print, etc.) and construct the scanner profile for prints corresponding to those matching samples. However, this will be a tedious and time consuming process for the user who has to spend time to lay down the respective test targets by identifying the printing technology of his/her document and then build the correction profile.
In order to solve these aforementioned problems the present invention automatically identifies the original is medium from which the electronic image was scanned, and thus permits a more consistent refinement of the accuracy of the color image reproduced.
The present invention is a device correction method and apparatus for color correcting a scanned electronic image.
The first embodiment is an image scanning apparatus that comprises a scanner and a spectrophotometric sensor that interacts logically with the scanner. The scanner scans a medium containing a color image, and the spectrophotometric sensor interacts with the scanner to aid in the automatic selection of a scanner color correction profile corresponding to the medium being scanned.
The second embodiment is an image scanning apparatus that comprises a scanner having different color correction scanning profiles and a spectrophotometric sensor that interacts logically with the scanner. When the scanner scans a medium containing a color image, the spectrophotometric sensor interacts with the scanner to aid in an automatic identification and a selection of at least one color correction scanning profile corresponding to the medium and the color image on the medium being scanned.
The third embodiment is a method of color correcting a scanner for a particular medium that comprises several steps. First, the medium is scanned to produce an electronic image. Second, a set of one or more regions of interest is determined in the electronic image. Third scanned values are extracted within the regions of interest. Fourth, at least one color value measurement is performed of at least one location on the image with a sensor interacting logically with the scanning device. Fifth, a scanner color correction is derived for the medium from the at least one color value measurement.
The fourth embodiment is a method of color correcting a scanner system for a particular medium, comprising several steps. First, a scanner stores a correction image electronically in memory. Second, at least one region of interest is determined in the scanner correction image. Third, a scanner correction image is printed on a medium with a printing machine. Fourth, the medium having scanner correction image is conveyed to a document scanner for scanning the scanner correction image. Fifth, the printed correction image on the medium is scanned to produce a scanned electronic image. Sixth, scanned values are extracted in the regions of interest. Seventh, at least one color value measurement is performed of the at least one region of interest on the scanner correction image with a sensor interacting logically with the scanning device. Eighth, a scanner color correction is derived for the medium from the at least one color value measurement.